我有一个非常简单的JFrame。有三个主要面板：顶部的横幅图像，左侧的按钮列表，以及用户将输入其登录信息以访问应用程序其余部分的主面板。
我正在使用GridBagLayout，尽管大多数人都像瘟疫一样避开它，但它对我来说非常简单，尽管它确实添加了许多行代码。但是我遇到了这个奇怪的问题，其中顶行（横幅图像）与底行（按钮和登录面板）重叠。我已经检查过并仔细检查并在网上查找答案，但无法弄清楚我做错了什么。
基本上，底行在整个JFrame中垂直居中，而不是在第二个GridBag行中。尽管事先被添加到屏幕上，但不知何故，BannerPanel正在被绘制。我认为它可能与BannerPanel的工作方式有关，但我找不到解决方法。
这就是它的样子：
https://sphotos-a.xx.fbcdn.net/hphotos-snc7/375898_3720190211823_1073177291_n.jpg
这应该是它的样子：
https://sphotos-a.xx.fbcdn.net/hphotos-ash4/314993_3720190291825_1429407717_n.jpg
这是我的代码：
public class LoginWindow extends JFrame implements ActionListener {
final static String unlockCode = "unlock";
ArrayList <User> userlist = new ArrayList <User> ();
User user = null;
// The visible parts of the window
GridBagConstraints gridbag;
JLabel inputLabel, errorLabel, lockedLabel, unlockLabel;
JTextField usernameField, unlockField;
JPasswordField passwordField;
JPanel inputPanel, usernamePanel, passwordPanel, unlockPanel;
public static void main(String[] args) {
LoginWindow win = new LoginWindow ();
win.userlist.add(new User ("username", "password", true));
}
public LoginWindow () {
setTitle("Login");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
getContentPane().setBackground(Color.GRAY);
setSize(640, 480);
setResizable(false);
resetGridbag();
// This is where I declare all the JLabels, JPanels, etc
inputLabel = new JLabel ("Secure Login");
inputLabel.setFont(new Font ("SansSerif", Font.BOLD, 24));
inputLabel.setForeground(Color.WHITE);
JLabel usernameLabel = new JLabel ("Username ");
usernameLabel.setForeground(Color.WHITE);
usernameField = new JTextField (10);
usernameField.setActionCommand("Login");
usernameField.addActionListener(this);
usernamePanel = new JPanel ();
usernamePanel.setBackground(Color.GRAY);
usernamePanel.add(usernameLabel);
usernamePanel.add(usernameField);
JLabel passwordLabel = new JLabel ("Password ");
passwordLabel.setForeground(Color.WHITE);
passwordField = new JPasswordField (10);
passwordField.setActionCommand("Login");
passwordField.addActionListener(this);
passwordPanel = new JPanel ();
passwordPanel.setBackground(Color.GRAY);
passwordPanel.add(passwordLabel);
passwordPanel.add(passwordField);
errorLabel = new JLabel ("");
errorLabel.setForeground(Color.WHITE);
lockedLabel = new JLabel ("You've been locked out!");
lockedLabel.setForeground(Color.WHITE);
unlockLabel = new JLabel ("Unlock Code");
unlockLabel.setForeground(Color.WHITE);
unlockField = new JTextField (10);
unlockField.setActionCommand("Unlock");
unlockField.addActionListener(this);
unlockPanel = new JPanel ();
unlockPanel.setBackground(Color.GRAY);
unlockPanel.add(unlockLabel);
unlockPanel.add(unlockField);
JLabel newPassword = new JLabel ("Request a new password");
newPassword.setForeground(Color.WHITE);
JPanel optionPanel = new JPanel ();
optionPanel.setBackground(Color.GRAY);
optionPanel.add(newPassword);
inputPanel = new JPanel ();
inputPanel.setBackground(Color.GRAY);
inputPanel.setLayout(new GridBagLayout ());
// Now I'm going to add them all to the screen
GridBagLayout gbl = new GridBagLayout ();
gbl.columnWeights = new double [] {0.0f, 1.0f};
gbl.rowWeights = new double [] {0.0f, 1.0f};
setLayout(gbl);
gridbag.gridwidth = 2;
gridbag.gridy = 0;
gridbag.fill = GridBagConstraints.HORIZONTAL;
add(new BannerPanel (), gridbag);
gridbag.gridy = 1;
gridbag.gridwidth = 1;
gridbag.anchor = GridBagConstraints.NORTHWEST;
add(optionPanel, gridbag);
gridbag.gridx++;
gridbag.anchor = GridBagConstraints.CENTER;
add(inputPanel, gridbag);
redraw();
setVisible(true);
}
public void resetGridbag () {
gridbag = new GridBagConstraints ();
gridbag.anchor = GridBagConstraints.CENTER;
gridbag.gridx = gridbag.gridy = 0;
}
public void reset () {
inputPanel.removeAll();
resetGridbag();
validate();
repaint();
}
public void redraw () {
reset();
if (user == null || !user.locked()) {
inputPanel.add(inputLabel, gridbag);
gridbag.gridy++;
inputPanel.add(new JLabel (" "), gridbag);
gridbag.gridy++;
inputPanel.add(usernamePanel, gridbag);
gridbag.gridy++;
inputPanel.add(passwordPanel, gridbag);
gridbag.gridy++;
inputPanel.add(new JLabel (" "), gridbag);
gridbag.gridy++;
inputPanel.add(errorLabel, gridbag);
}
else {
inputPanel.add(lockedLabel, gridbag);
gridbag.gridy++;
inputPanel.add(unlockPanel, gridbag);
gridbag.gridy++;
inputPanel.add(errorLabel, gridbag);
errorLabel.setText("");
}
validate();
repaint();
}
public void actionPerformed (ActionEvent e) {
String button = e.getActionCommand();
if (button.equals("Login")) {
boolean usernameMatch = false;
boolean passwordMatch = false;
for (int i = 0; i < userlist.size(); i++) {
if (usernameField.getText().equals(userlist.get(i).username())) {
usernameMatch = true;
user = userlist.get(i);
}
if (new String (passwordField.getPassword()).equals(userlist.get(i).password()))
passwordMatch = true;
}
passwordField.setText("");
if (usernameMatch) {
if (passwordMatch) {
user.unlock();
//new MainWindow ();
dispose();
}
else {
user.loginFail();
if (!user.locked())
errorLabel.setText("Login unsuccessful. " +
user.loginAttempts() + " attempts left!");
else
redraw();
}
}
else
errorLabel.setText("Login unsuccessful.");
validate();
}
else if (button.equals("Unlock")) {
if (unlockField.getText().equals(unlockCode)) {
errorLabel.setText("");
user.unlock();
redraw();
}
else {
errorLabel.setText("Invalid unlock code.");
validate();
}
unlockField.setText("");
}
}
}
class BannerPanel extends JPanel {
Image image;
int width = 0, height = 0;
double ratio = 0.0;
public BannerPanel () {
try {
image = ImageIO.read(BannerPanel.class
.getClassLoader().getResourceAsStream("banner.png"));
}
catch (Exception e) { e.printStackTrace(); }
}
@Override
protected void paintComponent (Graphics g) {
super.paintComponent(g);
ratio = (double) getWidth() / image.getWidth(null);
width = getWidth();
height = getImageHeight();
setSize(width, height);
if (image != null) {
g.drawImage(image, 0, 0, width, height, this);
}
}
public int getImageHeight () {
return (int) (image.getHeight(null) * ratio);
}
}
public class User {
String username = "";
String password = "";
boolean superuser = false;
int loginAttempts = 3;
public User (String username, String password, boolean superuser) {
this.username = username;
this.password = password;
this.superuser = superuser;
}
public String username () {
return username;
}
public String password () {
return password;
}
public boolean superuser () {
return superuser;
}
public int loginAttempts () {
return loginAttempts;
}
public void loginFail () {
if (loginAttempts > 0)
loginAttempts--;
}
public boolean locked () {
return (loginAttempts == 0);
}
public void lock () {
loginAttempts = 0;
}
public void unlock () {
loginAttempts = 3;
}
}
答案 0 :(得分：2)
BannerPanel的高度取决于图片的高度（height = getImageHeight();）。但是，这是BannerPanel 绘制的高度，而不是要求的高度。您需要覆盖getPreferredSize()以根据图像和比例提供正确的所需高度 - 否则将为BannerPanel假设0高度进行布局。
我认为问题在于您尝试创建一个高度和宽度之间固定比例的组件，同时将宽度推迟到父容器。这会导致您必须等待布局执行一次才能知道组件的首选大小。正如您所经历的那样，在paintComponent中执行这些计算将等待布局，调整大小，绘图，等待布局和再次绘制。这不是理想的 - 您必须执行两次布局，但没有内在的需要两次绘图。我不认为Swing能给你足够的控制来可靠地避免这种情况，但 是更多惯用的方法！例如，您可以覆盖setBounds以根据实际宽度更改首选高度：
@Override
public void setBounds(int x, int y, int width, int height) {
super.setBounds(x, y, width, height);
int pWidth = getPreferredSize().width;
setPreferredSize(new Dimension(pWidth, width / 2));
}
答案 1 :(得分：0)
Jacob Raihle有正确的想法，但它对我不起作用。
在setPreferredSize()旁边添加了对paintComponent() setSize()的调用的工作做了什么。
显然setSize()允许绘制整个图像，而setPreferredSize()告诉布局的其余部分它占用了多少空间。将这一切都用在一种方法而不必覆盖但是嘿，你会做什么会很棒？
这是我的BannerPanel对象的新代码：
class BannerPanel extends JPanel {
Image image;
int width = 0, height = 0;
double ratio = 0.0;
public BannerPanel () {
try {
image = ImageIO.read(BannerPanel.class
.getClassLoader().getResourceAsStream("banner.png"));
}
catch (Exception e) { e.printStackTrace(); }
}
@Override
protected void paintComponent (Graphics g) {
super.paintComponent(g);
ratio = (double) getWidth() / image.getWidth(null);
width = getWidth();
height = getImageHeight();
setPreferredSize(new Dimension (width, height));
setSize(width, height);
if (image != null) {
g.drawImage(image, 0, 0, width, height, this);
}
}
public int getImageHeight () {
return (int) (image.getHeight(null) * ratio);
}
}